GaN-based HEMTs (high electron mobility transistors) with a uniform AlGaN buffer, also commonly referred to as an AlGaN back-barrier, have been proposed as an alternative to the common HEMT on pure GaN buffer with the main aim to reduce punch-through effects and reduce source-to-drain leakage current in off-state conditions. Uniform AlGaN back-barriers also have been used in conjunction with normally-off device concepts, with the additional purpose of increasing the device threshold voltage as an effect of the additional quantum confinement of carriers between the main AlGaN barrier and the AlGaN back-barrier.
The reduction of the maximum electric field in presence of a uniform AlGaN back-barrier is mainly due to hole accumulation in off-state conditions. Accumulated holes effectively act as a back field-plate which shields the high voltage applied at the drain side and dramatically reduces the electric field in the gate region. However, while hole accumulation is very effective in reducing the maximum electric field in the device, this is very detrimental for device performance. Indeed, hole accumulation results in a reduction of the effective buffer thickness which in turn translates into a large increase in the device capacitance and consequently degradation of device performance. Greater device capacitance is detrimental not only because it implies a general increase in the overall device output capacitance, but also because it implies higher risks of unwanted spurious turn-on effects. In the typical case of a GaN HEMT with a uniform back-barrier, the Al content in the back-barrier is only a few percent (e.g. 3-4%). As a consequence, the effect of hole accumulation and breakdown reduction is only marginal. On the other hand, if the uniform back-barrier is to be used for reducing the overall electric field, then the Al content must be increased which results in the issues mentioned above.